We extend the classical forbidden-interval theorems for a
stochastic-resonance noise benefit in a nonlinear system to a quantum-optical
communication model and a continuous-variable quantum key distribution model.
Each quantum forbidden-interval theorem gives a necessary and sufficient
condition that determines whether stochastic resonance occurs in quantum
communication of classical messages. The quantum theorems apply to any quantum
noise source that has finite variance or that comes from the family of
infinite-variance alpha-stable probability densities. Simulations show the
noise benefits for the basic quantum communication model and the
continuous-variable quantum key distribution model.Comment: 13 pages, 2 figure